Many human retinal diseases cause vision loss by partial to complete destruction of the vascular layers of the eye that include the choroid and choriocapillaris, both of which nourish the outer anatomical retina and a portion of the inner anatomical retina of the eye.
Many other retinal diseases cause vision loss due to partial to complete degeneration of one or both of the two anatomical retinal layers directly, due to inherent abnormalities of these layers. The components of the retinal layers include Bruch's membrane and retinal pigment epithelium which comprise the “outer anatomical retinal layer”, and the photoreceptor, outer nuclear, outer plexiform, inner nuclear, inner plexiform, amacrine cell, ganglion cell and nerve fiber layers which comprise the “inner anatomical retinal layer”, also known as the “neuroretina”. The outer portion of the neuroretina is comprised of the photoreceptor and bipolar cell layers and is also known as the “outer retina” which is to be distinguished from the “outer anatomical retinal layer” as defined above. Loss of function of the outer retina is commonly the result of dysfunction of the outer anatomical retinal layer that provides nourishment to the outer retina and/or direct defects of the outer retina itself. The final common result is dysfunction of the outer retina that contains the light sensing cells, the photoreceptors. These “outer retina” diseases include age-related macula degeneration, retinitis pigmentosa, choroidal disease, long-term retinal detachment, diabetic retinopathies, Stargardt's disease, choroideremia, Best's disease, and rupture of the choroid. The inner portion of the neuroretina, however, often remains functionally and anatomically quite intact and may be activated by the appropriate stimuli.
There are currently numerous efforts underway to develop prosthetic devices that may be used to replace some degree of visual function to patients with the diseases described above. Many of the approaches are premised on the hypothesis that acute electrical stimulation using an array of stimulation electrodes may be used to form patterned vision. Typically, these approaches rely on relatively complex systems in which a video camera or similar device is used to capture images for subsequent processing and encoding. The encoded information thereafter controls electrical stimulation provided via an array of electrodes implanted proximate to retinal tissues. Typically, the electrode array is implanted epiretinally (on the ganglion cell layer side of the neuroretina) or subretinally (between the outer retina and the outer anatomical retinal layer, as defined above) and is connected through a wired or wireless connection to the appropriate control circuitry. In some instances, the wired connection must traverse the sclera, the tough outer coating of the eye often referred to as the white portion of the eye. Regardless, an advantage of providing such a connection between the control circuitry and the stimulating array is the ability to control the level of electrical stimulation delivered to neural tissues.
However, in addition to being relatively complex, systems of the type described above fail to take advantage of the eye's natural movements and its ability to focus images on the retina. They instead produce stimulation in a pattern not necessarily having any relationship to the eye's spatial orientation.
One approach that does take advantage of the eye's natural movements and focusing ability is the ASR® device developed by Optobionics Corporation. Comprising an array of several thousand electrode-tipped, independent photodiodes, the ASR® device is implanted in the subretinal space of the eye. The photodiodes are designed to essentially mimic the function of missing or non-functioning photoreceptors by converting incident light to electrical stimulation that may be further processed by the remaining retinal cell layers. Because of its simple design, the ASR® device offers several advantages over other, more complex retinal prosthesis systems. While it is believed that the ASR® device can be configured to generate sufficient electrical stimulation to reach stimulation thresholds, thereby inducing neuronal responses, efficacy of the device could be enhanced and perhaps better controlled through the provision of additional power.
Therefore, it would be advantageous to provide a retinal prosthesis that combines the simplicity and ability to exploit natural eye movements of the ASR® device with the power control capability of other, more complex retinal prosthesis systems.